fix: reset pooled NSScanner RPC responses to avoid stale usage data

NSScanner responses stream either a dataUsageEntry update or the
final dataUsageCache. The grid framework recycles stream responses
into a pool without resetting them, and the handler set only one
field per response - so a recycled response still carried the Final
(or Update) payload of an earlier scan. A remote node reading such a
response accepted the stale final cache, corrupting bucket usage
data across scanner cycles.

Honor Recycler types when pooling stream responses, implement it for
nsScannerResp, and clear the unset field at the response
construction sites for defense in depth.

Signed-off-by: Julien Laurenceau <julien.laurenceau@pepitedata.com>

cmd/storage-rest-common.go

@@ -83,3 +83,13 @@ type nsScannerResp struct {
 	Update *dataUsageEntry `msg:"u"`
 	Final  *dataUsageCache `msg:"f"`
 }
+
+// Recycle resets the response, so that recycled responses cannot
+// leak stale scanner data into a subsequent scan.
+func (r *nsScannerResp) Recycle() {
+	if r == nil {
+		return
+	}
+	r.Update = nil
+	r.Final = nil
+}

cmd/storage-rest-server.go

@@ -290,6 +290,9 @@ func (s *storageRESTServer) NSScannerHandler(ctx context.Context, params *nsScan
 		for update := range updates {
 			resp := storageNSScannerRPC.NewResponse()
 			resp.Update = &update
+			// Pooled responses may carry fields from a previous
+			// scan, always clear the fields not being set.
+			resp.Final = nil
 			out <- resp
 		}
 	}()
@@ -301,6 +304,7 @@ func (s *storageRESTServer) NSScannerHandler(ctx context.Context, params *nsScan
 	// Send final response.
 	resp := storageNSScannerRPC.NewResponse()
 	resp.Final = &ui
+	resp.Update = nil
 	out <- resp
 	return nil
 }

cmd/storage-rest-server_test.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2015-2023 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import "testing"
+
+// TestNSScannerRespPooling verifies that recycled NSScanner stream
+// responses cannot leak stale fields from a previous scan into new
+// responses, which would otherwise produce incorrect bucket usage
+// data.
+func TestNSScannerRespPooling(t *testing.T) {
+	dirty := storageNSScannerRPC.NewResponse()
+	dirty.Update = &dataUsageEntry{}
+	dirty.Final = &dataUsageCache{}
+	storageNSScannerRPC.PutResponse(dirty)
+
+	resp := storageNSScannerRPC.NewResponse()
+	if resp.Update != nil || resp.Final != nil {
+		t.Fatalf("pooled nsScannerResp leaked stale fields: Update %v, Final %v", resp.Update, resp.Final)
+	}
+}

internal/grid/handlers.go

@@ -698,6 +698,11 @@ func (h *StreamTypeHandler[Payload, Req, Resp]) PutRequest(r Req) {
 // These should be returned by the caller.
 func (h *StreamTypeHandler[Payload, Req, Resp]) PutResponse(r Resp) {
 	if r != h.nilResp {
+		if rc, ok := any(r).(Recycler); ok {
+			// Recycled stream responses must not leak fields
+			// from a previous use into the next response.
+			rc.Recycle()
+		}
 		//nolint:staticcheck // SA6002 IT IS A GENERIC VALUE! (and always a pointer)
 		h.respPool.Put(r)
 	}